1. Field of the Invention
The present invention relates to an alternator suitable for use in a vehicle.
2. Description of Related Art
A rotation speed during an idling state (hereinafter, referred to as idling engine speed) of a vehicle has a tendency to be set low in order to protect terrestrial environment such by decreasing exhausting gas and reducing an amount of fuel consumption. For example, there is such a vehicle that the idling engine speed thereof is set to around 550 rpm.
In a low friction engine like this, when the idling engine speed is reduced, a slight disturbance causes a flutter of an engine rotation speed. That is, a hunting phenomenon occurs because the engine is extraordinarily sensitive to the reduction of the rotation speed.
A mechanism of the hunting phenomenon will be explained in more detail.
FIG. 8 shows a relationship between rotation speed of a generator and torque required for driving thereof (hereinafter, referred to as driving torque). A curved line A1-C1 indicates a driving torque characteristic while a constant generation load is applied. A curved line A2-C2 indicates a driving torque characteristic while a constant generation load slightly larger than the constant generation load described above is applied. Points C1 and C2 indicate lower limit of the rotation speed and the driving torque for allowing the alternator to generate an output more than a required load. Consumption torque (load torque) of the alternator is suddenly changed at the points C1 and C2, and becomes the disturbance cannot be negligible for an engine. When the alternator is moved to lower rotation side than the points C1 and C2, since a current supply to a vehicle load cannot be satisfied by only output power of the alternator, a vehicle battery compensates the electric power.
For example, in a current supply state of a continuous service load (electric load required at least in term of vehicle operation such as ignition, drive of fuel injection valve, and driving power of control computer thereof) an engine becomes a stabilized state at point A1 in a range of the idling engine speed. At this time, when additional electric load, for example, a small light or the like is applied, an output of the alternator is increased by increasing in an exciting current. As a result, the driving torque of the alternator is increased, and an operational point moves from the point A1 to point A2. At this time, an engine only injects fuel to become the stabilized state in the point A1, a feedback control of fuel injection for an engine is remarkably delayed. Therefore, the rotation speed of the engine is reduced, and the operational point moves to point B after all. As the rotation speed is reduced, the driving torque for driving the alternator is increased, so that the operational point furthermore continues the lowering below the point B.
At this time, a feedback control system of fuel injection immediately detects the lowering of the rotation speed, and increases the amount of fuel injection for the purpose of maintaining the operational point to the point A2. However, since it takes a certain time from the fuel injection through an explosion thereof and conversion thereof into torque, thereby introducing remarkable control delay after all as described above.
Due to this control delay, the operational point drops to point D, discharge from the battery is caused. In a short while, the feedback control of the fuel injection for the engine starts to operate and the rotation speed transfers from the lowering to an uprising, while the alternator charges the battery in order to compensate a discharged capacity of the battery occurred at the previous time. At this time, the alternator has to output a total current of a current required by the electric load of the vehicle and charging current of the battery, and the operational point moves on a curved line D-E-F via point E by increasing the driving torque in an acceleration manner.
After a while, when the operational point reaches point F and a charge to the battery is finished, since output current of the alternator is reduced to a required electric load of the vehicle. Thus, the driving torque is also abruptly reduced and the operational point moves on a curved line F-G. At this time, the engine increases fuel for the purpose of increasing the rotation speed reduced to the point D. However, when the operational point exceeds the point F, the load torque of the engine is abruptly decreased. Thus, the rotation speed of the engine is increased in an acceleration manner, and the operational point is moved upwardly to point G. Thus, the feedback control system of the fuel injection detects the uprising of the rotation of the engine, and tries to control for the purpose of maintaining the operational point to the point A2 by reducing the amount of fuel injection. However, due to fluctuation of the driving torque and the control delay of the alternator, the engine rotation speed is fluctuated in a range of N1 through N2, and repeats hunting. FIG. 9 shows a timing chart showing the hunting phenomenon.
The hunting of the rotation speed of the alternator is improved by restraining the discharge of the battery by mounting a large output alternator. However, an adoption of the large output alternator is not easy viewing from a mounting space, cost, and a noise.
For inhibiting the hunting, JP-A-54-7111 discloses a method of adjusting the driving torque by controlling an output voltage corresponding to a speed differential of the alternator. JP-B2-6-55040 discloses a method for increasing an adjusting voltage while maintaining a delay of predetermined time with respect to the uprising of the rotation speed of an engine. JP-A-7-123796 discloses a method for detecting an application of the electric load, lowering the adjust voltage for a moment, and thereafter, the adjust voltage is gradually returned to the original state thereof.
However, in JP-A-54-7111, the control is started after the rotation speed starts to rise due to delay in the detection of a first drop of the rotation speed, so that a convergence of the hunting takes a lot of time, or it is resulted in an engine stop at the worst situation when the drop of the rotation speed is large. Further, according to the present method, an exciting current (or duty ratio) is controlled in order to properly control the output voltage of the alternator to a predetermined voltage. However, since the output voltage of the alternator connected to the battery depends on a state of the battery not on the exciting current, the control of the output voltage is not easy. For example, the output voltage of the alternator on a curved line A2-C2 charges the battery by generating power more than the open terminal voltage of the battery. However, when the rotation speed is reduced and the operational point is reduced under the point C2, the battery starts to discharge, and the terminal voltage of the battery continues to decrease in order to cancel out polarization obtained by the charge. At this time, the adjust voltage of the alternator is set at a constant value, so that the alternator itself cannot control the output voltage. Therefore, the output voltage depends on the terminal voltage of the battery. When the operational point resides on a curved line D-F, the dependence on the battery is the same.
In the method of the JP-B2-6-55040, the adjust voltage is changed in a considerably wide range from the idling engine speed to the maximum rotation speed. Viewed from paying attention to only the idling state, the battery voltage hardly changes. Therefore, the charge and discharge of the battery causing the hunting cannot be prevented. If the change of the adjust voltage is made large in the vicinity of the idling engine speed, the battery voltage in the maximum rotation speed becomes higher than necessary voltage, and the high voltage of the battery not only promotes a reduction in battery electrolyte and a decrease of battery life span, but also results in a deviation from a voltage range for appropriate operation of each device in the electric system.
According to the method of JP-A-7-123796, the application of electric load is detected, and the adjust voltage is reduced for a moment and is gradually returned to the original state. In this method, there arises the following disadvantage.
That is, in this method, in order to detect the application of the electric load only during engine idle without the delay, any signals to inform the application of the electric load from the vehicle must be received, and it becomes difficult to take a countermeasure within the alternator. An electrical system for a vehicle becomes complicated and expensive as in JP-A-6-343300.
An object of the present invention is to provide a vehicle alternator reducing and smoothening the fluctuation of driving torque thereof, so that the hunting of the rotation speed is suppressed during an engine idle.
According to the present invention, an AC generation section has a rotor including a plurality of field poles and driven by a vehicle engine, a field coil for magnetizing the field poles, an armature including an armature coil and arranged facing the field poles. A rectifying means rectifies AC voltage generated by the armature coil to DC voltage and charges an electricity accumulating means. A rotation speed detecting means detects an electricity amount related to rotation speed of the rotor. A voltage control unit includes a comparing section for comparing an electricity amount related to the DC voltage with a predetermined adjust voltage and a switch connected in series to the field coil. The voltage control unit converges the DC voltage to the adjust voltage by intermittently controlling the switch based on a compared result of the comparing section. The voltage control unit includes a shift means for shifting a rotation speed region to a low rotation speed side based on the electricity amount related to rotation speed. Within the rotation speed region, driving torque of the alternator suddenly changes as a result of an increase of duty ratio of said switch due to a shortage of generation capacity caused by a reduction of the rotation speed.
Thereby, a inflection point of a fluctuation of the engine rotation speed is forcibly moved, within an engine idling range, to rotation speed further lower than a lower limit to which an engine rotation speed drops, so that engine load torque is smoothly changed. Here, the engine rotation speed drop is caused by an increase of the driving torque (load torque viewed from engine) of the vehicle alternator particularly when an electric load is applied.